1. Field of the Invention
The present invention relates to an improvement of a holding mechanism for holding, for example, a semiconductor wafer while the wafer is being polished.
2. Description of the Related Art
Conventionally, in relation to a technique for polishing one face of a wafer of silicon, which is a typical semiconductor material, there has been known a polishing method and apparatus as disclosed, for example, in Japanese Patent Application Laid-Open (kokai) No. 5-74749. In the method and apparatus, a wafer is placed on a polishing turn table, such that the wafer surface to be polished is in contact with polishing pad attached onto the top surface of the polishing turn table. While the wafer is pressed downward by a top ring, the polishing turn table is moved along a small circular path to thereby cause the wafer to be polished by the polishing pad. In order to ensure that the wafer is polished over the entire area thereof under a uniform pressure, the wafer is pressed via a fluid. That is, an elastic membrane is attached onto the bottom surface of the top ring, and the fluid is confined in a fluid confinement space formed above the elastic membrane. Thus, while the elastic membrane is brought in close contact with the wafer, a pressing force is applied to the wafer to thereby press the wafer under a uniform pressure over the entire area thereof.
This conventional polishing method solves problems involved in a so-called wax method in which a wafer is attached onto a glass plate provided on the bottom surface of the top ring by using wax and processed. That is, since the elastic membrane absorbs ruggedness of the wafer surface, there is prevented the formation of dimples during polishing which would otherwise be caused by dust or the like caught between the glass plate and the wafer, the work and cost of applying wax are eliminated, and the work and cost of removing wax from the wafer and the glass plate after polishing are also eliminated.
However, the above-described technique does not have a function of finely adjusting the volume or pressure of the fluid confined in the fluid confinement space. As a result, the elastic membrane fails to assume a precisely flat shape, resulting in a failure to closely contact the wafer over the entire area thereof. This insufficient contact between the elastic membrane and the wafer causes an applied pressure to vary depending on a position on the wafer surface while the wafer surface is being polished, resulting in a failure to uniformly polish the wafer surface over the entire area thereof. For example, the amount of polishing differs between the peripheral area and the central area on the wafer surface, resulting in excessive removal or insufficient removal of material at the peripheral portion of the polished wafer.
Also, due to an insufficient force of holding a wafer, when a polishing rate is increased too much, a shift of the wafer results. This restrains the polishing rate, and thus a polishing efficiency is relatively low.
Thus, there has been eager demand for measures for finely adjusting the volume or pressure of a confined fluid to thereby establish close contact between an elastic membrane and a wafer over the entire area of the wafer so as to uniformly apply a pressure to the entire wafer surface, as well as for maintaining the wafer at a predetermined position even when the polishing rate is increased.